Abstract Cylindrical heat pipes with sintered-grooved composite wicks are manufactured by more than 20 processes. Essential to their thermal performances are the working fluid filling and vacuuming processes. In this work, the effects of various process parameters on the thermal performance of a composite heat pipe were examined experimentally by conducting transient and steady-state tests. Under the conditions of the first vacuuming process, the effective working length showed a more remarkable effect on the start-up performance of the heat pipes than the first vacuuming time and filling ratio. The isothermal performance demonstrated sensitivity to the filling ratio. Under the conditions of the second vacuuming process, the second vacuuming temperature showed a remarkable effect on the isothermal performance. The thermal resistances were less than 0.02 K/W at the evaporator and less than 0.09 K/W at the condenser with respect to those less than 0.16 K/W after the first vacuuming process.